The present invention relates to copolymer dispersions (copolymer polyols) having excellent stability and low viscosity, and to polyurethanes prepared by reacting these dispersions with reactive polyisocyanates.
Polyurethanes constitute a broad class of polymeric materials having a wide range of physical characteristics. The polymers are produced by the reaction of a polyisocyanate with a polyfunctional compound having an active hydrogen in its structure. This active hydrogen compound is generally a liquid or solid capable of being melted at a relatively low temperature. Most commonly, the active hydrogen compound contains hydroxyl groups as the moieties having the active hydrogen and thus are termed "polyols," e.g., the polyols of polyesters, polyamides, polyethers, or mixtures of two or more of such materials.
Although a wide variety of physical and chemical properties are obtainable by the proper selection of a polyisocyanate and the polyol as well as the conditions under which the polyurethane reaction is carried out, the resulting polyurethane often exhibits properties, for example, load bearing and processability, unacceptable for some applications.
To improve such properties, it has been the practice of the art to employ graft copolymer dispersions (often called "copolymer polyols or polymer polyols") prepared from vinyl monomers and polyols as shown in U.S. Pat. Nos. 3,383,351; 3,304,273; 3,523,093; 3,652,639 and 3,823,201. In 3,304,273, a mixture of styrene and a polar monomer are copolymerized while dispersed in a polyol which is essentially free of ethylenic unsaturation. Unfortunately, stable dispersions cannot be prepared by this technique if high proportions of styrene monomer are employed. Subsequently, in order to overcome this problem, it was found desirable to employ polyols which contain a significant amount of ethylenic unsaturation. As is shown in U.S. Pat. No. 3,823,201, such unsaturation is advantageously introduced by reacting the polyol with an organic compound having both ethylenic unsaturation and hydroxyl, carboxyl or epoxy moieties which are reactive with the active hydrogen moiety of the polyol.
While these improved graft copolymer dispersions solve many of the problems confronting the polyurethane art, other significant problems remain. Specifically, having a low viscosity polymer polyol is an important factor for urethane processing. Low viscosity allows greater loading of solids in the dispersion so that greater reinforcement is obtained in the final product. Low viscosity can also mean better mixing so that a more uniform product is obtained, shorter cycle times are required, and less expensive mixing equipment is needed. Compounding the problem of viscosity is the fact that added unsaturation, which is introduced to the polyol for colloidal stability, can significantly increase viscosity. In high solids formulations, the addition of highly reactive vinyl double bonds to the polyol can cause unworkable viscosities. Viscosity is more of a problem with "all acrylonitrile" dispersions than with styrene-acrylontrile dispersions. Indeed, in some instances dispersions having particles polymerized from essentially only acrylonitrile (or similar monomers) are so viscous as to be useless in many applications.
Accordingly, it would be highly desirable to provide an improved copolymer dispersion which at a given percent solids has a low viscosity.